EP4049381A1

EP4049381A1 - Phase error compensation for downlink systems with four correlated and uncalibrated antennas

Info

Publication number: EP4049381A1
Application number: EP19794697.3A
Authority: EP
Inventors: Jianguo Long; Shaohua Li
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2019-10-23
Filing date: 2019-10-23
Publication date: 2022-08-31
Also published as: CN114600386A; WO2021079173A1; US20220416862A1; US11894898B2

Abstract

A method, network node and wireless device (WD) for phase error compensation for Fifth Generation (5G) downlink systems with four correlated uncalibrated antennas are provided. A method includes applying N incremented phase rotations to one of the four antennas to produce N channel state information reference signal (CSI-RS) resources, N being an integer greater than 1. The method also includes transmitting N CSI-RS on 4 CSI-RS ports to a wireless device, WD, each of the N CSI-RS resources being transmitted with a different one of the incremented phase rotations. The method further includes receiving from the WD a CSI-RS resource indication (CRI) that indicates a particular one of the N CSI-RS resources. The method also includes applying, in subsequent transmissions to the WD, a phase rotation to the one of the four antennas, the phase rotation corresponding to the indicated CSI-RS resource.

Description

^{PHASE ERROR COMPENSATION FOR DOWNLINK SYSTEMS WITH} ^{FOUR CORRELATED AND UNCALIBRATED ANTENNAS}

^{TECHNICAL FIELD} ^{Wireless communication and in particular, to phase error compensation for} ^{Fifth Generation (5G) downlink systems with four correlated and uncalibrated} ^antennas.

^BACKGROUND ^{Fifth Generation (SG) wireless networking, also referred to as New Radio} ^{(NR), is defined by wireless communication standards developed by the Third} ^{Generation Partnership Project (3GPP), and is replacing the extant Long Term} ^{Evolution (LTE) wireless standard. An example of a typical antenna system used in} ^{LTE, and likely to be used in NR low band, is depicted in FIG. 1. FIG. 1 shows 4} ^{correlated downlink transmit antennas (numbered 0-3). The four antennas are cross-} ^{polarized, i.e., the antennas are either placed with a slant angle 45° (polarization A) or} ^{-45° (polarization B). Two cross-polarized antenna pairs are closely spaced with 0.5 to} ^{1λ separation. An advantage of such a configuration is that it provides excellent} ^{beamforming gain because the co-polarized antennas (antenna pair 0 and 1 or antenna} ^{pair 2 and 3) are correlated; and at the same time, it also allows for reasonable} ^{multiplexing gain of up to 4 layers resulting from a combination of polarization} ^{diversity and sufficient spatial diversity.} ^{Beamforming with correlated antennas requires that the phase difference} ^{between individual antenna elements be small. Any antenna error that affect phase} ^{relations could prevent systems from realizing full beamforming potential. Ideally, to} ^{achieve beamforming gain, the antennas shown in FIG. 1 should be calibrated.} ^{However, due to cost, most of 4 element antennas currently used in LTE base station} ^{(eNB) implementations are uncalibrated. As the wireless industry evolves into 5G,} ^{those radio-antenna systems will be reused. When antennas are uncalibrated, the} ^{signal over each antenna will have different phase (p} _k ^{, k = 0,1, 2, 3.} ^{For each pair of correlated co-polarized antenna pair of FIG. 1, i.e., antenna} ^{pair 0 and 1 for polarization A or antenna pair 2 and 3 for polarization B, the main} ^{lobe of the radiation pattern or beam during transmission points in the direction where} ^{the phases of antenna signals are added constructively. Hence, beam direction} ^{depends on the phase difference between two co-polarized antennas. When the phase} ^{difference between two correlated antennas change, the beam direction will change as} ^{illustrated by FIG. 2.} ^{The phase difference between antennas in each co-polarized antenna pair can} ^{be expressed approximately as:} ^and

^{If the antennas are calibrated, i.e., φ}k ^{= 0 for all k = 0,1, 2, 3, then Ø} _A ^{= Ø} _B ^{= 0 and} ^{the beams from two polarizations are aligned and point to bore sight, as illustrated} ^{by the dashed line in FIG. 2.} ^{If the antennas are not calibrated, i.e., φ}k ^{≠ 0 for all k = 0,1, 2, 3, but the} ^{phase differences of two polarizations are the same, i.e., Ø} _A ^{= Ø} _B ^{≠ 0, the beams} ^{from two polarizations are still aligned while beam direction will be deviated from} ^{bore sight. For example, when Ø} _A ^{= Ø} _B ^{≠ 135°, the beams of two polarizations can} ^{be illustrated by the solid line in FIG. 2.} ^{However, when the phase difference from two beams are not equal, i.e.,} ^Ø _A ^{≠ Ø} _B ^{, two beams will point to different directions. The example shown in FIG. 1} ^{can be considered such as a case when Ø} _A ^{= 0° and Ø} _A ^{= 135°.} ^{If the precoder codebook contains all combinations of phase compensations} ^{for the two co-polarized antenna pairs, this beam misalignment can be corrected by} ^{selecting a correct precoder. However, this is not the case for the NR type I codebook} ^{which assumes beams from two polarization always point to the same direction.} ^{Hence, for systems with four correlated and uncalibrated antennas, the NR type I} ^{codebook will have poor performance due to the beam misalignment caused by signal} ^{phase error.} ^{The 5G NR type I codebook assumes beams from two polarizations always} ^{point to the same direction. However, this is not the case for systems with four} ^{correlated and uncalibrated antennas as phase error of antennas can cause beam} ^{misalignment. Performance will be poor when applying the NR type I codebook} ^{directly to systems with four correlated and uncalibrated antennas.}

^SUMMARY ^{Some embodiments advantageously provide a method, network node and} ^{wireless device for phase error compensation for Fifth Generation (5G) downlink} ^{systems with four correlated and uncalibrated antennas.} ^{Some embodiments provide:} ^{• A method to introduce phase rotations to transmitted CSI-RS signals;} ^{• A method to detect the best quantized phase compensations for each} ^{WD in the systems by configuring the WD to report a CSI-RS resource indication} ^{(CRI) in a CSI report; and/or} ^{• A method to apply phase compensation to downlink signals.} ^{According to one aspect, a method in a network node for compensation of} ^{phase error between antennas of a four-antenna dual-polarization antenna array of a} ^{radio of the network node is provided. The method includes applying N incremented} ^{phase rotations to one of the four antennas to produce N channel state information} ^{reference signal (CSI-RS) resources, N being an integer greater than 1. The method} ^{further includes transmitting N CSI-RS on 4 CSI-RS ports to a wireless device, WD,} ^{each of the N CSI-RS resources being transmitted with a different one of the} ^{incremented phase rotations. The method also includes receiving from the WD a CSI-} ^{RS resource indication, CRI, indicating a particular one of the N CSI-RS resources.} ^{The method also includes applying, in subsequent transmissions to the WD, a phase} ^{rotation to the one of the four antennas, the phase rotation corresponding to the} ^{indicated CSI-RS resource.} ^{According to this aspect, in some embodiments, the N incremented phase} ^{rotations are achieved by multiplying a precoder codebook by a diagonal matrix} ^{whose diagonal elements are equal to one, except for one diagonal element} ^{corresponding to the one of the four antennas and being equal to exp(j2jm/N), where n} ^{is an integer from zero to N- 1. In some embodiments, each of N increments of the N} ^{incremented phase rotations are equal. In some embodiments, the particular one of} ^{the N CSI-RS resources is one having a highest spectrum efficiency of the N CSI-RS} ^resources. ^{According to another aspect, a network node configured to compensate for} ^{phase error between antennas of a four-antenna dual-polarization antenna array of a} ^{radio of the network node is provided. The network node includes processing} ^{circuitry configured to apply N incremented phase rotations to one of the four} ^{antennas to produce N channel state information reference signal (CSI-RS) resources,} ^{N being an integer greater than 1. The network node also includes a radio interface in} ^{communication with the processing circuitry, the radio interface configured to: (1)} ^{transmit N CSI-RS on 4 CSI-RS ports to a wireless device, WD, each of the N CSI-} ^{RS resources being transmitted with a different one of the incremented phase rotations} ^{and (2) receive from the WD a CSI-RS resource indication, CRI, indicating a} ^{particular one of the N CSI-RS resources. In these embodiments, the processing} ^{circuitry is further configured to apply, in subsequent transmissions to the WD, a} ^{phase rotation to the one of the four antennas, the phase rotation corresponding to the} ^{indicated CSI-RS resource.} ^{According to this aspect, in some embodiments, the N incremented phase} ^{rotations are achieved by multiplying a precoder codebook by a diagonal matrix} ^{whose diagonal elements are equal to one, except for one diagonal element} ^{corresponding to the one of the four antennas and being equal to exp(j2jm/N), where n} ^{is an integer from zero to N-l. In some embodiments, each of N increments of the N} ^{incremented phase rotations are equal. In some embodiments, the particular one of the} ^{N CSI-RS resources is one having a highest spectrum efficiency of the N CSI-RS} ^resources. ^{According to yet another aspect, a method in a WD includes receiving a signal} ^{from a network node, the signal having N channel state information reference signals,} ^{CSI-RS, transmitted by a four-antenna dual polarization antenna array of a radio of} ^{the network node, the signal generated by the network node by applying N} ^{incremented phase rotations to one of the four antennas to produce N channel state} ^{information reference signal (CSI-RS) resources, N being an integer greater than 1.} ^{The method also includes determining a CSI-RS of N CSI-RS resulting in a highest} ^{spectrum efficiency of all the N CSI-RS, and transmitting to the network node a CSI} ^{resource indicator, CRI, indicating the determined CSI-RS resulting in the highest} ^{spectrum efficiency of all the N CSI-RS.} ^{According to this aspect, in some embodiments, the WD further receives a} ^{signal configuring the WD to determine the CSI-RS with the highest spectrum} ^{efficiency of all the N CSI-RS received in the signal from the network node. In some} ^{embodiments, each increment of the N incremented phase rotations are equal. In some} ^{embodiments, the N incremented phase rotations are achieved by multiplying a} ^{precoder codebook by a diagonal matrix whose diagonal elements are equal to one,} ^{except for one diagonal element corresponding to one of the four antennas and being} ^{equal to exp(j2jm/N), where n is an integer from zero to N- 1.} ^{According to another aspect, a WD includes a radio interface configured to} ^{receive a signal from a network node, the signal having N channel state information} ^{reference signals, CSI-RS, transmitted by a four-antenna dual polarization antenna} ^{array of a radio of the network node, the signal generated by the network node by} ^{applying N incremented phase rotations to one of the four antennas to produce N} ^{channel state information reference signal (CSI-RS) resources, N being an integer} ^{greater than 1. The WD also includes processing circuitry in communication with the} ^{radio interface, the processing circuitry configured to determine a CSI-RS of the N} ^{CSI-RS resulting in a highest spectrum efficiency of all the N CSI-RS. The radio} ^{interface is further configured to transmit to the network node a CSI resource} ^{indicator, CRI, indicating the determined CSI-RS resulting in the highest spectrum} ^{efficiency of all the N CSI-RS.} ^{According to this aspect, in some embodiments, the WD further receives a} ^{signal configuring the WD to determine the CSI-RS with the highest spectrum} ^{efficiency of all the N CSI-RS received in the signal from the network node. In some} ^{embodiments, each increment of the N incremented phase rotations are equal. In some} ^{embodiments, the N incremented phase rotations are achieved by multiplying a} ^{precoder codebook by a diagonal matrix whose diagonal elements are equal to one,} ^{except for one diagonal element corresponding to one of the four antennas and being} ^{equal to exp(j2jm/N), where n is an integer from zero to N- 1.}

^{BRIEF DESCRIPTION OF THE DRAWINGS} ^{A more complete understanding of the present embodiments, and the attendant} ^{advantages and features thereof, will be more readily understood by reference to the} ^{following detailed description when considered in conjunction with the accompanying} ^{drawings wherein:} ^{FIG. 1 depicts a four element cross polarized antenna;} ^{FIG. 2 depicts a beam patter shown a result of phase error,} ^{FIG. 3 is a schematic diagram of an exemplary network architecture} ^{illustrating a communication system according to the principles in the present} ^disclosure; ^{FIG. 4 is a block diagram of a network node in communication with a wireless} ^{device over a wireless connection according to some embodiments of the present} ^disclosure; ^{FIG. 5 is a flowchart of an exemplary process in a network node for phase} ^{error compensation for Fifth Generation (5G) downlink systems with four correlated} ^{and uncalibrated antennas;} ^{FIG. 6 is a flowchart of an exemplary process in a wireless device for phase} ^{error compensation for Fifth Generation (5G) downlink systems with four correlated} ^{and uncalibrated antennas.} ^{FIG. 7 is a flowchart of an exemplary process for phase error compensation} ^{for Fifth Generation (5G) downlink systems with four correlated and uncalibrated} ^{antennas; and} ^{FIG. 8 is a diagram depicting beam patters for successive CSI-RS.}

^{DETAILED DESCRIPTION} ^{Before describing in detail exemplary embodiments, it is noted that the} ^{embodiments reside primarily in combinations of apparatus components and} ^{processing steps related to phase error compensation for Fifth Generation (5G)} ^{downlink systems with four correlated and uncalibrated antennas. Accordingly,} ^{components have been represented where appropriate by conventional symbols in the} ^{drawings, showing only those specific details that are pertinent to understanding the} ^{embodiments so as not to obscure the disclosure with details that will be readily} ^{apparent to those of ordinary skill in the art having the benefit of the description} ^herein. ^{As used herein, relational terms, such as “first” and “second,” “top” and} ^{“bottom,” and the like, may be used solely to distinguish one entity or element from} ^{another entity or element without necessarily requiring or implying any physical or} ^{logical relationship or order between such entities or elements.} ^{The terminology used herein is for the purpose of describing particular} ^{embodiments only and is not intended to be limiting of the concepts described herein.} ^{As used herein, the singular forms “a”, “an” and “the” are intended to include the} ^{plural forms as well, unless the context clearly indicates otherwise. It will be further} ^{understood that the terms “comprises,” “comprising,” “includes” and/or “including”} ^{when used herein, specify the presence of stated features, integers, steps, operations,} ^{elements, and/or components, but do not preclude the presence or addition of one or} ^{more other features, integers, steps, operations, elements, components, and/or groups} ^thereof. ^{In embodiments described herein, the joining term, “in communication with”} ^{and the like, may be used to indicate electrical or data communication, which may be} ^{accomplished by physical contact, induction, electromagnetic radiation, radio} ^{signaling, infrared signaling or optical signaling, for example. One having ordinary} ^{skill in the art will appreciate that multiple components may interoperate, and} ^{modifications and variations are possible of achieving the electrical and data} ^{communication.} ^{In some embodiments described herein, the term “coupled,” “connected,” and} ^{the like, may be used herein to indicate a connection, although not necessarily} ^{directly, and may include wired and/or wireless connections.} ^{The term “network node” used herein can be any kind of network node} ^{comprised in a radio network which may further comprise any of base station (BS),} ^{radio base station, base radio interface station (BTS), base station controller (BSC),} ^{radio network controller (RNC), g Node B (gNB), evolved Node B (eNB or eNodeB),} ^{Node B, multi-standard radio (MSR) radio node such as MSR BS, multi-cell/multicast} ^{coordination entity (MCE), relay node, donor node controlling relay, radio access} ^{point (AP), transmission points, transmission nodes, Remote Radio Unit (RRU)} ^{Remote Radio Head (RRH), a core network node (e.g., mobile management entity} ^{(MME), self-organizing network (SON) node, a coordinating node, positioning node,} ^{MDT node, etc.), an external node (e.g., 3rd party node, a node external to the current} ^{network), nodes in distributed antenna system (DAS), a spectrum access system} ^{(SAS) node, an element management system (EMS), etc. The network node may also} ^{comprise test equipment. The term “radio node” used herein may be used to also} ^{denote a wireless device (WD) such as a wireless device (WD) or a radio network} ^node. ^{In some embodiments, the non-limiting terms wireless device (WD) or a user} ^{equipment (UE) are used interchangeably. The WD herein can be any type of wireless} ^{device capable of communicating with a network node or another WD over radio} ^{signals, such as wireless device (WD). The WD may also be a radio communication} ^{device, target device, device to device (D2D) WD, machine type WD or WD capable} ^{of machine to machine communication (M2M), low-cost and/or low-complexity WD,} ^{a sensor equipped with WD, Tablet, mobile terminals, smart phone, laptop embedded} ^{equipped (LEE), laptop mounted equipment (LME), USB dongles, Customer} ^{Premises Equipment (CPE), an Interet of Things (IoT) device, or a Narrowband IoT} ^{(NB-IOT) device etc.} ^{Also, in some embodiments the generic term “radio network node” is used. It} ^{can be any kind of a radio network node which may comprise any of base station,} ^{radio base station, base radio interface station, base station controller, network} ^{controller, RNC, evolved Node B (eNB), Node B, gNB, Multi-cell/multicast} ^{Coordination Entity (MCE), relay node, access point, radio access point, Remote} ^{Radio Unit (RRU) Remote Radio Head (RRH).} ^{As used herein, a “radio interface” may also be referred to as a radio, and may} ^{include an antenna array.} ^{Note that although terminology from one particular wireless system, such as,} ^{for example, 3GPP LTE and/or New Radio (NR), may be used in this disclosure, this} ^{should not be seen as limiting the scope of the disclosure to only the aforementioned} ^{system. Other wireless systems, including without limitation Wide Band Code} ^{Division Multiple Access (WCDMA), Worldwide Interoperability for Microwave} ^{Access (WiMax), Ultra Mobile Broadband (UMB) and Global System for Mobile} ^{Communications (GSM), may also benefit from exploiting the ideas covered within} ^{this disclosure.} ^{Note further, that functions described herein as being performed by a wireless} ^{device or a network node may be distributed over a plurality of wireless devices} ^{and/or network nodes. In other words, it is contemplated that the functions of the} ^{network node and wireless device described herein are not limited to performance by} ^{a single physical device and, in fact, can be distributed among several physical} ^devices. ^{Unless otherwise defined, all terms (including technical and scientific terms)} ^{used herein have the same meaning as commonly understood by one of ordinary skill} ^{in the art to which this disclosure belongs. It will be further understood that terms} ^{used herein should be interpreted as having a meaning that is consistent with their} ^{meaning in the context of this specification and the relevant art and will not be} ^{interpreted in an idealized or overly formal sense unless expressly so defined herein.} ^{Some embodiments are directed to phase error compensation for Fifth} ^{Generation (5G) downlink systems with four correlated and uncalibrated antennas.} ^{According to one aspect, a method includes applying N incremented phase rotations} ^{to one of the four antennas to produce N channel state information reference signal} ^{(CSI-RS) resources, N being an integer greater than 1. The method also includes} ^{transmitting N CSI-RS on 4 CSI-RS ports to a wireless device, WD, each of the N} ^{CSI-RS resources being transmitted with a different one of the incremented phase} ^{rotations. The method further includes receiving from the WD a CSI-RS resource} ^{indication, CRI, indicating which CSI-RS resource provides a highest spectrum} ^{efficiency of the N CSI-RS resources. The method also includes applying, in} ^{subsequent transmissions to the WD, a phase rotation to the one of the four antennas,} ^{the phase rotation corresponding to the indicated CSI-RS resource.} ^{Referring again to the drawing figures, in which like elements are referred to} ^{by like reference numerals, there is shown in FIG. 3 a schematic diagram of a} ^{communication system 10, according to an embodiment, such as a 3GPP-type cellular} ^{network that may support standards such as LTE and/or NR (5G), which comprises an} ^{access network 12, such as a radio access network, and a core network 14. The access} ^{network 12 comprises a plurality of network nodes 16a, 16b, 16c (referred to} ^{collectively as network nodes 16), such as NBs, eNBs, gNBs or other types of} ^{wireless access points, each defining a corresponding coverage area 18a, 18b, 18c} ^{(referred to collectively as coverage areas 18). Each network node 16a, 16b, 16c is} ^{connectable to the core network 14 over a wired or wireless connection 20. A first} ^{wireless device (WD) 22a located in coverage area 18a is configured to wirelessly} ^{connect to, or be paged by, the corresponding network node 16a. A second WD 22b in} ^{coverage area 18b is wirelessly connectable to the corresponding network node 16b.} ^{While a plurality of WDs 22a, 22b (collectively referred to as wireless devices 22) are} ^{illustrated in this example, the disclosed embodiments are equally applicable to a} ^{situation where a sole WD is in the coverage area or where a sole WD is connecting} ^{to the corresponding network node 16. Note that although only two WDs 22 and} ^{three network nodes 16 are shown for convenience, the communication system may} ^{include many more WDs 22 and network nodes 16.} ^{Also, it is contemplated that a WD 22 can be in simultaneous communication} ^{and/or configured to separately communicate with more than one network node 16} ^{and more than one type of network node 16. For example, a WD 22 can have dual} ^{connectivity with a network node 16 that supports LTE and the same or a different} ^{network node 16 that supports NR. As an example, WD 22 can be in communication} ^{with an eNB for LTE/E-UTRAN and a gNB for NR/NG-RAN.} ^{A network node 16 (eNB or gNB) is configured to include a phase rotation} ^{unit 24 which is configured to applying N incremented phase rotations to one of the} ^{four antennas to produce N channel state information reference signal (CSI-RS)} ^{resources. A wireless device 22 is configured to include a CSI selector unit 26 which} ^{is configured to determining a CSI-RS of N CSI-RS resulting in a highest spectrum} ^{efficiency of all the N CSI-RS.} ^{Example implementations, in accordance with an embodiment, of the WD 22} ^{and, network node 16 discussed in the preceding paragraphs will now be described} ^{with reference to FIG. 4.} ^{The communication system 10 includes a network node 16 including hardware} ^{28 enabling it to communicate with the WD 22. The hardware 28 may include a radio} ^{interface 30 for setting up and maintaining at least a wireless connection 31 with a} ^{WD 22 located in a coverage area 18 served by the network node 16. The radio} ^{interface 30 may be formed as or may include, for example, one or more RF} ^{transmitters, one or more RF receivers, one of more RF transceivers and/or one or} ^{more RF radio interfaces. The radio interface 30 includes an array of antennas 32 to} ^{radiate and receive signal carrying electromagnetic waves. In example embodiments} ^{of this disclosure, the antenna array 32 has an array of 4 antenna elements, one} ^{example of which is 4 cross polarized antennas oriented as shown in FIG. 1.} ^{In the embodiment shown, the hardware 28 of the network node 16 further} ^{includes processing circuitry 34. The processing circuitry 34 may include a memory} ^{36 and a processor 38. In particular, in addition to or instead of a processor, such as a} ^{central processing unit, and memory, the processing circuitry 34 may comprise} ^{integrated circuitry for processing and/or control, e.g., one or more processors and/or} ^{processor cores and/or FPGAs (Field Programmable Gate Array) and/or ASICs} ^{(Application Specific Integrated Circuitry) adapted to execute instructions. The} ^{processor 38 may be configured to access (e.g., write to and/or read from) the} ^{memory 36, which may comprise any kind of volatile and/or nonvolatile memory,} ^{e.g., cache and/or buffer memory and/or RAM (Random Access Memory) and/or} ^{ROM (Read-Only Memory) and/or optical memory and/or EPROM (Erasable} ^{Programmable Read-Only Memory).} ^{Thus, the network node 16 further has software 40 stored internally in, for} ^{example, memory 36, or stored in external memory (e.g., database, storage array,} ^{network storage device, etc.) accessible by the network node 16 via an external} ^{connection. The software 40 may be executable by the processing circuitry 34. The} ^{processing circuitry 34 may be configured to control any of the methods and/or} ^{processes described herein and/or to cause such methods, and/or processes to be} ^{performed, e.g., by network node 16. Processor 38 corresponds to one or more} ^{processors 38 for performing network node 16 functions described herein. The} ^{memory 36 is configured to store data, programmatic software code and/or other} ^{information described herein. In some embodiments, the software 40 may include} ^{instructions that, when executed by the processor 38 and/or processing circuitry 34,} ^{causes the processor 38 and/or processing circuitry 34 to perform the processes} ^{described herein with respect to network node 16. For example, processing circuitry} ^{34 of the network node 16 may include phase rotation unit 24 which is configured to} ^{applying N incremented phase rotations to one of the four antennas to produce N} ^{channel state information reference signal (CSI-RS) resources.} ^{The communication system 10 further includes the WD 22 already referred to.} ^{The WD 22 may have hardware 42 that may include a radio interface 44 configured to} ^{set up and maintain a wireless connection 31 with a network node 16 serving a} ^{coverage area 18 in which the WD 22 is currently located. The radio interface 44 may} ^{be formed as or may include, for example, one or more RF transmitters, one or more} ^{RF receivers, one or more RF transceivers and/or one or more RF radio interfaces.}

^{The radio interface 44 includes an array of antennas 46 to radiate and receive signal} ^{carrying electromagnetic waves. The array of antennas 46 of the WD 22 may have 4} ^{antenna elements or fewer or greater than 4 antenna elements.} ^{The hardware 60 of the WD 22 further includes processing circuitry 48. The} ^{processing circuitry 48 may include a memory 50 and a processor 52. In particular, in} ^{addition to or instead of a processor, such as a central processing unit, and memory,} ^{the processing circuitry 48 may comprise integrated circuitry for processing and/or} ^{control, e.g., one or more processors and/or processor cores and/or FPGAs (Field} ^{Programmable Gate Array) and/or ASICs (Application Specific Integrated Circuitry)} ^{adapted to execute instructions. The processor 52 may be configured to access (e.g.,} ^{write to and/or read from) memory 50, which may comprise any kind of volatile} ^{and/or nonvolatile memory, e.g., cache and/or buffer memory and/or RAM (Random} ^{Access Memory) and/or ROM (Read-Only Memory) and/or optical memory and/or} ^{EPROM (Erasable Programmable Read-Only Memory).} ^{Thus, the WD 22 may further comprise software 54, which is stored in, for} ^{example, memory 50 at the WD 22, or stored in external memory (e.g., database,} ^{storage array, network storage device, etc.) accessible by the WD 22. The software} ^{54 may be executable by the processing circuitry 48. The software 54 may include a} ^{client application 56. The client application 56 may be operable to provide a service} ^{to a human or non-human user via the WD 22.} ^{The processing circuitry 48 may be configured to control any of the methods} ^{and/or processes described herein and/or to cause such methods, and/or processes to} ^{be performed, e.g., by WD 22. The processor 52 corresponds to one or more} ^{processors 52 for performing WD 22 functions described herein. The WD 22} ^{includes memory SO that is configured to store data, programmatic software code} ^{and/or other information described herein. In some embodiments, the software 54} ^{and/or the client application 56 may include instructions that, when executed by the} ^{processor 52 and/or processing circuitry 48, causes the processor 52 and/or processing} ^{circuitry 48 to perform the processes described herein with respect to WD 22. For} ^{example, the processing circuitry 48 of the wireless device 22 may include CSI} ^{selector unit 26 which is configured to determining a CSI-RS of N CSI-RS resulting} ^{in a highest spectrum efficiency of all the N CSI-RS.} ^{In some embodiments, the inner workings of the network node 16 and WD 22} ^{may be as shown in FIG. 4 and independently, the surrounding network topology may} ^{be that of FIG. 3.} ^{The wireless connection 31 between the WD 22 and the network node 16 is in} ^{accordance with the teachings of the embodiments described throughout this} ^{disclosure. More precisely, the teachings of some of these embodiments may improve} ^{the data rate, latency, and/or power consumption and thereby provide benefits such as} ^{reduced user waiting time, relaxed restriction on file size, better responsiveness,} ^{extended battery lifetime, etc. In some embodiments, a measurement procedure may} ^{be provided for the purpose of monitoring data rate, latency and other factors on} ^{which the one or more embodiments improve.} ^{Although FIGS. 3 and 4 show various “units” such as phase rotation unit 24} ^{and CSI selector unit 26 as being within a respective processor, it is contemplated that} ^{these units may be implemented such that a portion of the unit is stored in a} ^{corresponding memory within the processing circuitry. In other words, the units may} ^{be implemented in hardware or in a combination of hardware and software within the} ^{processing circuitry.} ^{FIG. 5 is a flowchart of an exemplary process in a network node 16 for phase} ^{error compensation for Fifth Generation (5G) downlink systems with four correlated} ^{and uncalibrated antennas. One or more blocks described herein may be performed} ^{by one or more elements of network node 16 such as by one or more of processing} ^{circuitry 34 (including the phase rotation unit 24), processor 38, and/or radio interface} ^{30. Network node 16 such as via processing circuitry 34 and/or processor 38 and/or} ^{radio interface 30 is configured to apply N incremented phase rotations to one of the} ^{four antennas to produce N channel state information reference signal (CSI-RS)} ^{resources, N being an integer greater than 1 (Block S134). The process includes} ^{transmitting N CSI-RS on 4 CSI-RS ports to a wireless device, WD, each of the N} ^{CSI-RS resources being transmitted with a different one of the incremented phase} ^{rotations (Block S 136). The process includes receiving from the WD a CSI-RS} ^{resource indication, CRI, indicating a particular one of the N CSI-RS resources} ^{(Block S 138). The process also includes applying, in subsequent transmissions to the} ^{WD, a phase rotation to the one of the four antennas, the phase rotation corresponding} ^{to the indicated CSI-RS resource (Block S140).} ^{FIG. 6 is a flowchart of an exemplary process in a wireless device 22} ^{according to some embodiments of the present disclosure. One or more blocks} ^{described herein may be performed by one or more elements of wireless device 22} ^{such as by one or more of processing circuitry 48 (including the CSI selector unit 26),} ^{processor 52 and/or radio interface 44. Wireless device 22 such as via processing} ^{circuitry 48 and/or processor 52 and/or radio interface 44 is configured to receive a} ^{signal from a network node, the signal having N channel state information reference} ^{signals, CSI-RS, transmitted by a four-antenna dual polarization antenna array of a} ^{radio of the network node, the signal generated by the network node by applying N} ^{incremented phase rotations to one of the four antennas to produce N channel state} ^{information reference signal (CSI-RS) resources, N being an integer greater than 1} ^{(Block S 142). The process also includes determining a CSI-RS of N CSI-RS} ^{resulting in a highest spectrum efficiency of all the N CSI-RS (Block S144). The} ^{process further includes transmitting to the network node a CSI resource indicator,} ^{CRI, indicating the determined CSI-RS resulting in the highest spectrum efficiency of} ^{all the N CSI-RS (Bloc S 146).} ^{FIG 7 is flowchart of an exemplary process involving both wireless device 22} ^{and network node 16. The network node 16 configures each WD 22 with a CSI-RS} ^{resource set with N CSI-RS resources (Block S 148). The network node 16 transmits,} ^{via the radio interface 30, N CSI resources with phase rotations on one antenna} ^{(Block S 150). The network node 16 configures WDs to send CSI report with CRI} ^{back to the network node 16 (Block S152). WD 22 sends CSI report with CRI (Block} ^{S154). The network node 16, via processing circuitry 34, applies phase rotation} ^{indicated by the CRI to PDSCH precoding (Block S156).} ^{Having described the general process flow of arrangements of the disclosure} ^{and having provided examples of hardware and software arrangements for} ^{implementing the processes and functions of the disclosure, the sections below} ^{provide details and examples of arrangements for phase error compensation for Fifth} ^{Generation (5G) downlink systems with four correlated and uncalibrated antennas.} ^{5G NR Type codebook}

^{The NR type 1 codebook specified by the 3GPP is based on a set of pre-} ^{defined precoding matrices. The precoding matrix, denoted as W, can be described as} ^{a two-stage precoding structure as follows:}

^{The first stage of the precoding structure, i.e., may be described as a} ^{codebook, and consists essentially a group of a two dimensional (2D) grid-of-beams} ^{(GoB), which may be characterized as:} ^{where w}h ^{and w} _v ^{are precoding vectors selected from an over-sampled discrete} ^{Fourier transform (DFT) for the horizontal direction and the vertical direction,} ^{respectively. The precoding vectors may be expressed by:} ^{where 0} ₁ ^{and 0} ₂ ^{are the over-sampling rate in vertical and horizontal directions,} ^{respectively.} ^{For 4 CSI-RS ports:} ^{The second stage of the precoding matrix, i.e., W} ₂ ^{, is used for beam selection} ^{within the group of the 2D GoB as well as the associated co-phasing between two} ^{polarizations. The co-phasing vector for one layer is defined as:}

^{The precoder matrix for one-layer transmission may be created by appending} ^{columns of one layer precoder vectors is defined as:}

^{The precoder matrix for multi-layer transmission may be created by appending} ^{columns of one layer precoder vectors as:} ^{where L is the number of layers.} ^{The co-phasing vector does not change the phase difference between co¬} ^{polarized antennas. The NR type I codebook assumes that the beams of two} ^{polarizations point to the same direction.} ^{Phase Compensation} ^{In case Ø} _A ^{≠ Ø} _B ^{, a phase rotation can be applied to one of the 4 antennas of} ^{the antenna array 32 to make the beams of two polarizations aligned. Without loss of} ^{generality, hereafter it is assumed that a phase rotation Δφ} ₃ ^{is applied to antenna} ^{number 3 of the 4 antennas, such as shown in FIG. 1:}

^{The phase rotation Δφ} ₃ ^{to make Ø} _A ^{and Ø} _B ^{equal is:}

^{If the phase rotation Δφ} ₃ ^{can be accurately measured, then this phase error can} ^{be compensated by applying to a CSI-RS with the following port-to-antenna mapping} ^matrix: ^{so that the WD 22 can report CSI such as for example via its own radio interface 44 to} ^{the network node 16, based on compensated CSI-RS signals. The physical downlink} ^{shared channel (PDSCH) beamforming weights may also be phase compensated} ^{based on the WD-reported rank indicator precoder matrix indicator (RIZPMI) and} ^{phase compensated CSI-RS:}

^{Note that W} _PMI ^{is the 3GPP defined NR type 1 codebook. Hence the PDSCH} ^{precoder disclosed above can be considered as an enhancement to the NR codebook} ^{for phase compensation.} ^{Accurate phase error measurement and compensation can be done by antenna} ^{calibration which requires additional hardware and signaling and consequently higher} ^{cost. The solution described here is a low cost solution which measures and} ^{compensates the phase error by software so that the phase error is smaller than a} ^{predefined threshold.} ^{Define a series of N equally incremented phase rotations to be applied to any} ^{of the four antennas of antenna array 32, the phase rotations being, for example,} ^{1. Again, without loss of generality, these phase rotations} m^{ay, once again, be applied to antenna 3 of the 4 element antenna array 32 as} ^{mentioned above.} ^{To identify the optimal phase compensation} C^{SI-RS resources of 4 CSI-RS ports will be transmitted. The CSI-RS resource n will} ^{be transmitted with a phase rotation N on the antenna 3 by using following port-} t^{o-antenna mapping for CSI-RS.}

^{The beam direction of polarization B with each phase rotation will change as} ^{illustrated by FIG. 8. Note that FIG. 8 is an example only, and embodiments are not} ^{limited to achieving the beam patterns shown in FIG. 8.} ^{The WD 22 will be configured to send a CSI report with the CSI-RS resource} ^{indication (CRI) which indicates the CSI-RS resource providing the highest spectrum} ^{efficiency i.e., the CSI-RS resource beamformed with the best W} _p2a ^: ^{Thus, according to one aspect, a method in a network node 16 for} ^{compensation of phase error between antennas of a four-antenna dual-polarization} ^{antenna array of a radio of the network node 16 is provided. The method includes} ^{applying, via the processing circuitry 34, N incremented phase rotations to one of the} ^{four antennas to produce N channel state information reference signal (CSI-RS)} ^{resources, N being an integer greater than 1. The method further includes} ^{transmitting, via the radio interface 30, N CSI-RS on 4 CSI-RS ports to a wireless} ^{device, WD 22, each of the N CSI-RS resources being transmitted with a different one} ^{of the incremented phase rotations. The method also includes receiving, via the radio} ^{interface 30, from the WD 22 a CSI-RS resource indication, CRI, indicating a} ^{particular one of the N CSI-RS resources. The method also includes applying, via the} ^{processing circuitry 34, in subsequent transmissions to the WD 22, a phase rotation to} ^{the one of the four antennas, the phase rotation corresponding to the indicated CSI-RS} ^resource. ^{According to this aspect, in some embodiments, the N incremented phase} ^{rotations are achieved by multiplying, via the processing circuitry 34, a precoder} ^{codebook by a diagonal matrix whose diagonal elements are equal to one, except for} ^{one diagonal element corresponding to the one of the four antennas and being equal to} ^{exp(j2Tm/N), where n is an integer from zero to N-l. In some embodiments, each of N} ^{increments of the N incremented phase rotations are equal. In some embodiments, the} ^{particular one of the N CSI-RS resources is one having a highest spectrum efficiency} ^{of the N CSI-RS resources.} ^{According to another aspect, a network node 16 configured to compensate for} ^{phase error between antennas of a four-antenna dual-polarization antenna array 43 of} ^{a radio of the network node 16 is provided. The network node 16 includes processing} ^{circuitry 34 configured to apply N incremented phase rotations to one of the four} ^{antennas to produce N channel state information reference signal (CSI-RS) resources,} ^{N being an integer greater than 1. The network node 16 also includes a radio interface} ^{30 in communication with the processing circuitry, the radio interface 30 configured} ^{to: (1) transmit N CSI-RS on 4 CSI-RS ports to a wireless device, WD 22, each of the} ^{N CSI-RS resources being transmitted with a different one of the incremented phase} ^{rotations and (2) receive from the WD 22 a CSI-RS resource indication, CRI,} ^{indicating a particular one of the N CSI-RS resources. In these embodiments, the} ^{processing circuitry 34 is further configured to apply, in subsequent transmissions to} ^{the WD 22, a phase rotation to the one of the four antennas, the phase rotation} ^{corresponding to the indicated CSI-RS resource.} ^{According to this aspect, in some embodiments, the N incremented phase} ^{rotations are achieved by multiplying, via the processing circuitry 34, a precoder} ^{codebook by a diagonal matrix whose diagonal elements are equal to one, except for} ^{one diagonal element corresponding to the one of the four antennas and being equal to} ^{exp(j2Tπn/N), where n is an integer from zero to N-l. In some embodiments, each of N} ^{increments of the N incremented phase rotations are equal. In some embodiments, the} ^{particular one of the N CSI-RS resources is one having a highest spectrum efficiency} ^{of the N CSI-RS resources.} ^{According to yet another aspect, a method in a WD 22 includes receiving, via} ^{the radio interface 44, a signal from a network node 16, the signal having N channel} ^{state information reference signals, CSI-RS, transmitted by a four-antenna dual} ^{polarization antenna array 32 of a radio interface 30 of the network node 16, the} ^{signal generated by the network node 16 by applying N incremented phase rotations} ^{to one of the four antennas of the antenna array 32 to produce N channel state} ^{information reference signal (CSI-RS) resources, N being an integer greater than 1.} ^{The method also includes determining, via the processing circuitry 48, a CSI-RS of N} ^{CSI-RS resulting in a highest spectrum efficiency of all the N CSI-RS, and} ^{transmitting, via the radio interface 44, to the network node 16 a CSI resource} ^{indicator, CRI, indicating the determined CSI-RS resulting in the highest spectrum} ^{efficiency of all the N CSI-RS.} ^{According to this aspect, in some embodiments, the WD 22 further receives,} ^{via the radio interface 44, a signal configuring the WD 22 to determine the CSI-RS} ^{with the highest spectrum efficiency of all the N CSI-RS received in the signal from} ^{the network node 16. In some embodiments, each increment of the N incremented} ^{phase rotations are equal. In some embodiments, the N incremented phase rotations} ^{are achieved by multiplying, via the processing circuitry 48, a precoder codebook by a} ^{diagonal matrix whose diagonal elements are equal to one, except for one diagonal} ^{element corresponding to one of the four antennas and being equal to exp(j2jcn/N),} ^{where n is an integer from zero to N-l.} ^{According to another aspect, a WD 22 includes a radio interface 44 configured} ^{to receive a signal from a network node 16, the signal having N channel state} ^{information reference signals, CSI-RS, transmitted by a four-antenna dual} ^{polarization antenna array 32 of the radio interface 30 of the network node 16, the} ^{signal generated by the network node 16 by applying N incremented phase rotations} ^{to one of the four antennas to produce N channel state information reference signal} ^{(CSI-RS) resources, N being an integer greater than 1. The WD 22 also includes} ^{processing circuitry 48 in communication with the radio interface 44, the processing} ^{circuitry 48 configured to determine a CSI-RS of the N CSI-RS resulting in a highest} ^{spectrum efficiency of all the N CSI-RS. The radio interface 44 is further configured} ^{to transmit to the network node 16 a CSI resource indicator, CRI, indicating the} ^{determined CSI-RS resulting in the highest spectrum efficiency of all the N CSI-RS.} ^{According to this aspect, in some embodiments, the WD 22 further receives,} ^{via the radio interface 44, a signal configuring the WD 22 to determine the CSI-RS} ^{with the highest spectrum efficiency of all the N CSI-RS received in the signal from} ^{the network node 16. In some embodiments, each increment of the N incremented} ^{phase rotations are equal. In some embodiments, the N incremented phase rotations} ^{are achieved by multiplying, via the processing circuitry 48, a precoder codebook by a} ^{diagonal matrix whose diagonal elements are equal to one, except for one diagonal} ^{element corresponding to one of the four antennas and being equal to exp(j2jcn/N),} ^{where n is an integer from zero to N-l.} ^{As will be appreciated by one of skill in the art, the concepts described herein} ^{may be embodied as a method, data processing system, and/or computer program} ^{product. Accordingly, the concepts described herein may take the form of an entirely} ^{hardware embodiment, an entirely software embodiment or an embodiment} ^{combining software and hardware aspects all generally referred to herein as a} ^{“circuit” or “module.” Furthermore, the disclosure may take the form of a computer} ^{program product on a tangible computer usable storage medium having computer} ^{program code embodied in the medium that can be executed by a computer. Any} ^{suitable tangible computer readable medium may be utilized including hard disks,} ^{CD-ROMs, electronic storage devices, optical storage devices, or magnetic storage} ^devices. ^{Some embodiments are described herein with reference to flowchart} ^{illustrations and/or block diagrams of methods, systems and computer program} ^{products. It will be understood that each block of the flowchart illustrations and/or} ^{block diagrams, and combinations of blocks in the flowchart illustrations and/or block} ^{diagrams, can be implemented by computer program instructions. These computer} ^{program instructions may be provided to a processor of a general purpose computer,} ^{special purpose computer, or other programmable data processing apparatus to} ^{produce a machine, such that the instructions, which execute via the processor of the} ^{computer or other programmable data processing apparatus, create means for} ^{implementing the functions/acts specified in the flowchart and/or block diagram block} ^{or blocks.} ^{These computer program instructions may also be stored in a computer} ^{readable memory or storage medium that can direct a computer or other} ^{programmable data processing apparatus to function in a particular manner, such that} ^{the instructions stored in the computer readable memory produce an article of} ^{manufacture including instruction means which implement the function/act specified} ^{in the flowchart and/or block diagram block or blocks.} ^{The computer program instructions may also be loaded onto a computer or} ^{other programmable data processing apparatus to cause a series of operational steps to} ^{be performed on the computer or other programmable apparatus to produce a} ^{computer implemented process such that the instructions which execute on the} ^{computer or other programmable apparatus provide steps for implementing the} ^{functions/acts specified in the flowchart and/or block diagram block or blocks.}

^{It is to be understood that the functions/acts noted in the blocks may occur out of the} ^{order noted in the operational illustrations. For example, two blocks shown in} ^{succession may in fact be executed substantially concurrently or the blocks may} ^{sometimes be executed in the reverse order, depending upon the functionality/acts} ^{involved. Although some of the diagrams include arrows on communication paths to} ^{show a primary direction of communication, it is to be understood that} ^{communication may occur in the opposite direction to the depicted arrows.} ^{Computer program code for carrying out operations of the concepts described} ^{herein may be written in an object oriented programming language such as Java® or} ^{C++. However, the computer program code for carrying out operations of the} ^{disclosure may also be written in conventional procedural programming languages,} ^{such as the "C" programming language. The program code may execute entirely on} ^{the user's computer, partly on the user's computer, as a stand-alone software package,} ^{partly on the user's computer and partly on a remote computer or entirely on the} ^{remote computer. In the latter scenario, the remote computer may be connected to the} ^{user's computer through a local area network (LAN) or a wide area network (WAN),} ^{or the connection may be made to an external computer (for example, through the} ^{Interet using an Interet Service Provider).} ^{Many different embodiments have been disclosed herein, in connection with} ^{the above description and the drawings. It will be understood that it would be unduly} ^{repetitious and obfuscating to literally describe and illustrate every combination and} ^{subcombination of these embodiments. Accordingly, all embodiments can be} ^{combined in any way and/or combination, and the present specification, including the} ^{drawings, shall be construed to constitute a complete written description of all} ^{combinations and subcombinations of the embodiments described herein, and of the} ^{manner and process of making and using them, and shall support claims to any such} ^{combination or subcombination.} ^{Abbreviations} ^Explanation ^AAS ^{Active Antenna System} ^CQI ^{Channel Quality Indicator} ^PDSCH ^{Physical Downlink Shared Channel} ^CSI-RS ^{Channel State Information Reference Signal} ^NZP-CSI-RS ^{Non-zero power CSI-RS} ^DPT ^{Discrete Fourier Transform} ^MU-MIMO ^{Multi-User MIMO} ^PMI ^{Precoding Matrix Indicator} ^RI ^{Rank Indication} ^SRS ^{Sounding Reference Signal} ^TTI ^{Transmission Time Interval} ^{It will be appreciated by persons skilled in the art that the embodiments} ^{described herein are not limited to what has been particularly shown and described} ^{herein above. In addition, unless mention was made above to the contrary, it should} ^{be noted that all of the accompanying drawings are not to scale. A variety of} ^{modifications and variations are possible in light of the above teachings without} ^{departing from the scope of the following claims.}

Claims

^{What is claimed is:}

^{1. A method in a network node (16) for compensation of phase error between} ^{antennas of a four-antenna dual-polarization antenna array of a radio of the network} ^{node (16), the method comprising:} ^{applying (S134) N incremented phase rotations to one of the four antennas to} ^{produce N channel state information reference signal (CSI-RS) resources, N being an} ^{integer greater than 1;} ^{transmitting (S136) N CSI-RS on 4 CSI-RS ports to a wireless device, WD} ^{(22), each of the N CSI-RS resources being transmitted with a different one of the} ^{incremented phase rotations;} ^{receiving (S138) from the WD (22) a CSI-RS resource indication, CRI,} ^{indicating a particular one of the N CSI-RS resources; and} ^{applying (S140), in subsequent transmissions to the WD (22), a phase rotation} ^{to the one of the four antennas, the phase rotation corresponding to the indicated CSI-} ^{RS resource.}

^{2. The method of Claim 1, wherein the N incremented phase rotations are} ^{achieved by multiplying a precoder codebook by a diagonal matrix whose diagonal} ^{elements are equal to one, except for one diagonal element corresponding to the one} ^{of the four antennas and being equal to exp(j2jm/N), where n is an integer from zero} ^{to N-1.}

^{3. The method of Claim 1, wherein each of N increments of the N incremented} ^{phase rotations are equal.}

^{4. The method of any of Claims 1-3, wherein the particular one of the N CSI-RS} ^{resources is one having a highest spectrum efficiency of the N CSI-RS resources.} ^{5. A network node (16) configured to compensate for phase error between} ^{antennas of a four-antenna dual-polarization antenna array (32) of a radio of the} ^{network node (16), the network node (16) comprising:} ^{processing circuitry (34) configured to apply N incremented phase rotations to} ^{one of the four antennas to produce N channel state information reference signal} ^{(CSI-RS) resources, N being an integer greater than 1;} ^{a radio interface (30) in communication with the processing circuitry (34), the} ^{radio interface (30) configured to:} ^{transmit N CSI-RS on 4 CSI-RS ports to a wireless device, WD (22),} ^{each of the N CSI-RS resources being transmitted with a different one of the} ^{incremented phase rotations; and} ^{receive from the WD (22) a CSI-RS resource indication, CRI,} ^{indicating a particular one of the N CSI-RS resources; and} ^{the processing circuitry (34) further configured to apply, in subsequent} ^{transmissions to the WD (22), a phase rotation to the one of the four antennas, the} ^{phase rotation corresponding to the indicated CSI-RS resource.} ^{6. The network node (16) of Claim 5, wherein the N incremented phase rotations} ^{are achieved by multiplying a precoder codebook by a diagonal matrix whose} ^{diagonal elements are equal to one, except for one diagonal element corresponding to} ^{the one of the four antennas and being equal to exp(j2jm/N), where n is an integer} ^{from zero to N-l.}

^7. ^{The network node (16) of Claim 5, wherein each of N increments of the N} ^{incremented phase rotations are equal.}

^{8. The network node (16) of any of Claims 5-7, wherein the particular one of the} ^{N CSI-RS resources is one having a highest spectrum efficiency of the N CSI-RS} ^resources.

^{9. A method in a wireless device, WD, (22), the method comprising:} ^{receiving (S142) a signal from a network node (16), the signal having N} ^{channel state information reference signals, CSI-RS, transmitted by a four-antenna} ^{dual polarization antenna array of a radio of the network node (16), the signal} ^{generated by the network node (16) by applying N incremented phase rotations to one} ^{of the four antennas to produce N channel state information reference signal (CSI-RS)} ^{resources, N being an integer greater than 1;} ^{determining (S144) a CSI-RS of N CSI-RS resulting in a highest spectrum} ^{efficiency of all the N CSI-RS; and} ^{transmitting (S146) to the network node (16) a CSI resource indicator, CRI,} ^{indicating the determined CSI-RS resulting in the highest spectrum efficiency of all} ^{the N CSI-RS.}

^{10. The method of Claim 9, wherein the WD (22) further receives a signal} ^{configuring the WD (22) to determine the CSI-RS with the highest spectrum} ^{efficiency of all the N CSI-RS received in the signal from the network node (16).}

^11. ^{The method of any of Claims 9 and 10, wherein each increment of the N} ^{incremented phase rotations are equal.}

^{12. The method of any of Claims 9 and 10, wherein the N incremented phase} ^{rotations are achieved by multiplying a precoder codebook by a diagonal matrix} ^{whose diagonal elements are equal to one, except for one diagonal element} ^{corresponding to one of the four antennas and being equal to exp(j2jcn/N), where n is} ^{an integer from zero to N-l.}

^{13. A wireless device, WD (22), the WD (22) comprising:} ^{a radio interface (44) configured to receive a signal from a network node (16),} ^{the signal having N channel state information reference signals, CSI-RS, transmitted} ^{by a four-antenna dual polarization antenna array (32) of a radio of the network node} ^{(16), the signal generated by the network node (16) by applying N incremented phase} ^{rotations to one of the four antennas to produce N channel state information reference} ^{signal (CSI-RS) resources, N being an integer greater than 1;} ^{processing circuitry (48) in communication with the radio interface (44), the} ^{processing circuitry (48) configured to determine a CSI-RS of the N CSI-RS resulting} ^{in a highest spectrum efficiency of all the N CSI-RS; and} ^{the radio interface (44) further configured to transmit to the network node (16)} ^{a CSI resource indicator, CRI, indicating the determined CSI-RS resulting in the} ^{highest spectrum efficiency of all the N CSI-RS.} ^{14. The WD (22) of Claim 9, wherein the WD (22) further receives a signal} ^{configuring the WD (22) to determine the CSI-RS with the highest spectrum} ^{efficiency of all the N CSI-RS received in the signal from the network node (16).}

^{15. The WD (22) of any of Claims 9 and 10, wherein each increment of the N} ^{incremented phase rotations are equal.}

^{16. The WD (22) of any of Claims 9 and 10, wherein the N incremented phase} ^{rotations are achieved by multiplying a precoder codebook by a diagonal matrix} ^{whose diagonal elements are equal to one, except for one diagonal element} ^{corresponding to one of the four antennas and being equal to exp(j2jcn/N), where n is} ^{an integer from zero to N-l.}